The use of Ziegler-Natta catalyst systems to promote various olefin polymerizations is well known. These catalyst systems function in both the gas phase, and slurry as well as solution polymerization processes. Of these processes, the gas phase and slurry polymerization processes are also known as particle form processes, socalled because the polymer is formed as discrete particles, the size and shape of which is a function of the size and shape of the catalyst particle. The polymer particle is thus said to replicate the initial catalyst particle. The final size of the polymer particle is a function of both the initial catalyst particle size and the productivity of the catalyst. Thus, in preparing a catalyst to be used in a gas-phase polymerization process, great care is taken in the catalyst precursor preparation in order to control both polymer particle size and morphology in addition to productivity. Examples of such Ziegler-Natta catalysts include those disclosed in U.S. Pat. Nos. 4,302,565; 4,482,687; 4,508,842; 4,990,479; 5,122,494; 5,290,745; and, 6,187,866.
Another polymer property that is desirably controlled through catalyst control is the particle size distribution, especially with respect to the low end of the distribution, as an unacceptable amount of small catalyst particles could lead to the generation of small polymer particles known as “polymer fines”. Polymer fines are undesirable in gas phase fluidized bed polymerization systems, as they tend to segregate to the top of the fluidizing bed, causing problems with bed level control. They are also preferentially entrained into the cycle gas leading to system plugging in heat exchangers and compressors, buildup in the bottom head of the reaction system and formation of gels due to continued polymerization at lower temperatures than the bulk of the polymer product. All of the above lead to poor commercial operation, reduced polymerization efficiency, and generally impaired operation. High levels of fines can also cause problems in downstream handling of the polymer once it exits the polymerization system. Fines can cause poor flow in purge bins, plug filters in bins and present safety problems. The above problems make elimination or reduction of polymer fines important to commercial operation of a gas-phase polymerization process.
In a multiple series reactor system, where the composition of the polymers produced in the separate reactors is often widely variant, the presence of polymer fines is particularly harmful to continuous and smooth operation. This is due to the extreme importance of precise bed level control as the product properties of the polymer are strongly influenced by the relative amount of polymer produced in each reactor. If the bed weights are not precisely known, it is extremely difficult to properly control the product exiting the final reactor.
With respect to the preparation of linear low density polyethylene and other ethylene/α-olefin copolymers, it is preferred to produce polymer in the separate reactors with both large molecular weight differences and relatively large differences in incorporated comonomer. To produce final polymers with the best physical properties, it is preferred to have one of the reactors produce a polymer with high molecular weight and incorporating a majority of the comonomer. In the second reactor, a low molecular weight portion of the polymer is formed which may also have comonomer incorporated, but normally in an amount less than that incorporated in the high molecular weight portion. When the high molecular weight component is produced first, polymer fines can become a significant problem, especially when the flow index (I21) of the resulting polymer is in the range from 0.1 to 2.0 g/10 min, and the incorporated comonomer content is less than 5 weight percent, especially less than 4.5 wt weight percent.
Depending on the order of production of the different polymers in the multiple reactor system (that is high molecular weight first, lower molecular weight second or the reverse), the fines will tend to have significantly different polymer properties than the bulk of the polymer granules. This is due to the fact that the fines also tend to be the youngest particles in the reactor and hence have had insufficient residence time in the reactor to produce a representative amount of polymer before transit to the second reactor in series.
This in turn leads to further problems in compounding of the polymer into pellets for end-use. In particular, the fines are normally of significantly different molecular weight or branching composition compared to the bulk polymer. Although the particles of both the bulk material and the fines will melt at roughly the same temperature, mixing is hampered unless the products have a similar isoviscous temperature (that is the temperature at which the melt viscosity of the two products is essentially the same). These polymer fines, which tend to be of significantly different molecular weight than the bulk of the polymer and differing isoviscous temperature, are then poorly mixed with the bulk phase. Upon cooling after pellet formation, these poorly mixed regions, if of sufficient size, will be visible in blown films as gels or in other extruded articles, resulting in visual defects and stress concentrators leading to premature failure of an article made therefrom.
Thus, polymer fines are, in general a problem for gas phase olefin polymerization processes and, in particular an issue for staged or series reactor systems in which precise control of polymer composition is only achieved by precise control of the relative amount of polymer produced in the multiple reactors.
Polymer fines can be removed from the polymerization reactor though use of a cyclone on the recycle line, however this reduces productivity and increases operating costs. In addition, the fines tend to be higher in catalyst concentration as they are, on average, younger particles. Removing these particles from the polymerization reactor increases the need for fresh catalyst further increasing costs. Since the polymer fines are still active for further polymerization, special care must be taken to make sure that the fines do not plug the cyclone. Any areas in which polymer particles can congregate in the presence of olefin can result in continued polymerization leading to formation of agglomerated particles and large chunks of polymer.
U.S. Pat. No. 5,969,061 disclosed the use of a solvent in an attempt to reduce polymer fines by making the bulk of the polymer particles stickier, resulting in the fines attaching to the larger particles. However, increasing polymer stickiness can result in further problems downstream in product separation and makes the reaction system more vulnerable to loss of recycle flow due to power failures, increasing the risk of large agglomerate formation. The addition of large amounts of solvent also increases the cost and complexity of the reaction system and requires apparatus for recycle of the solvent for reuse. It would be desirable to produce fewer fines during the polymerization reaction, thereby reducing the need for other polymer fines control systems.
In gas phase polymerization systems, it is known that, generally, each catalyst particle produces one polymer particle. Catalyst particles, in general increase in particle size proportionally to the cube root of the catalyst productivity. That is, the polymer particle size is expressed by the formula: polymer particle size=Constant×(Catalyst Productivity)1/3.
While not being bound by any one theory, it is believed that polymer fines originate either from fines in the catalyst or by particle attrition of the growing polymer. Given that fines can still be present in a polymer produced in the first reactor of a multiple reactor configuration producing tough, high mechanical strength, high molecular weight polymer, it is unlikely that particle attrition is the primary cause of polymer fines in such systems. Thus, catalyst particle fines are believed to be the predominant cause of polymer fines. Such catalyst fines can be removed by a variety of methods, ranging from eluting to sieving of the catalyst prior to use. This, however, adds both cost and complexity to the catalyst preparation process as well as increases the likelihood of catalyst contamination during the additional processing steps.
Operating the reaction system at higher levels of catalyst productivity can also reduce polymer fines. For single reactor systems, this is usually a feasible approach, however operating at catalyst productivity levels that are too high can result in operability problems due to polymer particle agglomeration. In extreme cases, higher levels of fines due to fracture of catalyst particles during polymerization may also result. For multiple reactor systems in which the catalyst is added only to the first reactor in the series, increasing catalyst productivity in the first reactor to minimize fines can result in the inability to run the second (or additional) reactors at commercially feasible conditions due to catalyst deactivation.
In order to compensate for this activity loss due to catalyst deactivation, the first reactor of the multiple reactor system is often operated in a “low productivity” regime so that there is sufficient catalyst activity remaining to complete polymerization in the second (and subsequent) reactors. However, the operation at lower catalyst productivity in the first reactor results in a reduction in polymer particle size, further increasing the need to control and reduce fines which might be caused by the catalyst.
As already explained, the particle size of a given polymer particle is a function of both the initial catalyst particle size, and the productivity of the catalyst; that is how much polymer grows from the initial catalyst particle during the polymerization process. Thus small particle sized polymer or polymer fines can be a result of either small initial catalyst particle size or low catalyst productivity, or both, and when both conditions are present, the generation of polymer fines is exacerbated.
The catalysts used in many olefin polymerization processes are of the Ziegler-Natta type. In particular, for gas phase polymerizations, the catalyst is often made from a precursor comprising magnesium and transition metal halides, particularly titanium chlorides in an electron donor solvent. This solution is often either deposited on a porous catalyst support, or a filler is added, which, on subsequent spray drying, provides additional mechanical strength to the particles. The solid particles from either method of production are often slurried in a diluent to produce a high viscosity mixture, which is then used in a gas-phase polymerization. Exemplary catalyst compositions are described in U.S. Pat. Nos. 6,187,866 and 5,290,745. Precipitated/crystallized catalyst compositions such as those described in U.S. Pat. Nos. 6,511,935 and 6,248,831, may also be used. Additional techniques for forming suitable catalyst precursors for use herein are disclosed in U.S. Pat. Nos.: 5,247,032, 5,247,031, 5,229,342, 5,153,158, 5,151,399, 5,146,028, 5,124,298, 5,106,806, 5,082,907, 5,077,357, 5,066,738, 5,066,737, 5,034,361, 5,028,671, 4,990,479, 4,927,797, 4,829,037, 4,816,433, 4,728,705, 4,548,915, 4,547,476, 4,540,679, 4,535,068, 4,472,521, 4,460,701, 4,442,276, and 4,330,649.
One advantage of the use of a spray drying process is that it allows the particle size and morphology of the catalyst, and hence the final product, to be controlled by variation of the process parameters of the spray dryer. Such parameters include the speed of the atomizer, the solids content of the slurry to be dried, the inlet and outlet gas temperatures of the dryer and the feed rate of the slurry to the atomizer.
However, due to the nature of spray drying, some small particles are always present. In particular, some “micro-fine” particles are formed during the spray drying process. These are also frequently called “daughter” particles and result from break up of droplets during the spray drying operation. These particles end up in the final spray dried catalyst composition and are of essentially the same chemical composition as the larger size, desired particles. These particles are seen in the <10 micron fraction of the particle size distribution of the catalyst and can form fine polymer particles that are the root of operational problems.
The catalyst precursor as produced is essentially inactive for olefin polymerization due to the presence of the Lewis Base electron donor. Activation of the catalyst precursor requires the removal of the electron donor from the vicinity of the active site, that is, the metal and, if necessary, reduction of the metal. The activator extracts the electron donor compound from the active site in one of several ways. The electron donor can be removed by complex formation, or by alkylation or by reduction and alkylation if the valence state of the metal requires reduction. Typical activating compounds are Lewis Acids. The activator is used to remove at least 90 percent, preferably all or as near to all as possible, of the electron donor from the active site, that is, the transition metal.
If the Lewis Acid is a non-reducing compound, such as BCl3, AlCl3, or similar chlorinating agent, a reducing compound, typically a trialkylaluminum an aluminum dialkyl halide may be added to fully activate the catalyst precursor. Precursors that are not fully halogenated will also require either use of a halogen donating Lewis Acid or a separate halogenation step prior to use.
Activation of the catalyst typically occurs in the polymerization reactor by the cocatalyst. However, complete activation of the catalyst inside the polymerization reactor typically requires a substantial excess of activator compound and in-the case of higher (C3, C4 and up) olefin polymerizations, reintroduction of a Lewis base as a selectivity control agent. Advantages to this technique are its simplicity of catalyst manufacture and feed. However, use of excess activator compound not only leads to added operational expense, but it may cause operational problems or detriment to the final product. Ultimately, large quantities of activator are required due to dilution by monomers, diluents, condensing agents, and other components within the reactor.
Partial pre-activation can occur prior to the polymerization reactor, however this additional step, because it potentially increases the exposure of the active catalyst to impurities and other deactivators, can cause a decrease in catalyst productivity especially on extended storage prior to use. Such a deactivation and loss of productivity would in turn be expected to cause an increase in polymer fines. Thus, as full activation of the catalyst is normally completed in the polymerization reactor with excess co-catalyst and generally occurs whether the catalyst has been partially activated or not, until now there has been no driving force to pursue partial pre-activation of the catalyst prior to addition to the reactor.
However, it would still be highly advantageous to have a process that would minimize the generation of polymer fines in a gas phase polymerization. It would also be advantageous if this process were to be applicable to a gas phase process utilizing multiple reactors. It would be even more advantageous if such a process involved a relatively simple manipulation of the catalyst rather than the more expensive and difficult process modifications such as cyclone operation or addition of solvents to the reactor. Finally, a process in which fully activated catalyst composition is supplied to the reactor would additionally be desirable.